Technology

Sputtered Blind Holes

A superior alternative to chemical PTH on aluminum backed PTFE substrates

Dr. K. Ramachandran, Filtran Microcircuits Inc.

Introduction

Aluminum backed PTFE substrates are finding increasing applications in microwave circuit design, especially for efficient packaging of higher power modules. Aluminum (Al) is not only much lighter than copper and brass, but has excellent machining properties necessary for fabricating intricate 3 dimensional shapes, making it ideally suited for aerospace applications.

Due to its extremely high electrochemical reactivity, however, Al is incompatible with most of the chemical processing used in printed circuit manufacturing. Any plating on Al has to be preceded by elaborate cleaning and surface treatments designed to form a barrier layer isolating the metal from the plating chemicals. There are two techniques to make Al platable by conventional means. The most common method called zincating consists of dipping the Al in a proprietary solution to form a thin coating of metallic zinc on all the surfaces. This is immediately followed by electroless nickel and then copper plating. The other method of plating on Al makes use of anodization in an electrolytic bath by passage of electric current. Even though an anodized film is non-conductive in the conventional sense, under appropriate conditions of grain growth formation, it provides nucleation sites for subsequent Cu plating. Internally generated electrostatic fields in the anodic film prevents the Cu plating solution from attacking the Al surface, while permitting the Cu plating to proceed as normal.

Compared with zincating, anodizing requires much tighter control of process parameters. However, the absence of a third metal (Zn) in the interface makes the process more reliable for PTH applications. Both these methods are quite satisfactory for generalized plating of Cu on flat Al surfaces. But when three dimensional features such as holes, cavities and pockets are encountered, the plating coverage tends to become unreliable. This severely restricts the aspect ratio (depth/diameter) of holes that can be successfully plated. For example, a hole of 50 mil diameter and 50 mil depth (aspect ratio = 1) would have unsatisfactory coverage inside the holes. If such an Al backed circuit board is processed using standard PTH chemicals, it would not only produce voids and blisters inside the holes, but could also poison the chemicals as they come into contact with the unprotected Al. Unless extremely fine tuned processing (zincating or anodizing) is employed, it is impractical to achieve reliable PTH in such cases.

One other approach that many board fabricators have successfully used is the insertion of tight fitting pins wherever plated through holes are required. These pins are made from materials such as brass, nickel and stainless steel, accurately machined and press fitted into holes passing through the top copper foil and the PTFE dielectric, firmly anchored into the aluminum body. After insertion, the pin is soldered onto the surrounding copper pad as part of the subsequent surface mount assembly of components. Considerable mechanical precision is required to achieve reliable PTH in this fashion. The integrity of such a mechanical joint between dissimilar materials is questionable under conditions of severe thermal shock, mechanical vibration and chemical corrosion. Furthermore, as the size gets smaller (<50 mil diameter) it becomes increasingly difficult to install the pins.

The New Approach

The processing sequence in the development of the sputtered blind hole is illustrated in Fig. 1. Vacuum metallization by magnetron sputtering has become a well-established technique in the semiconductor industry over the past decade. However, the high capital cost of equipment and the complexity of processing has precluded economical application of such methods in printed circuit board fabrication. At Filtran we have researched the technology of magnetron sputtering and adapted it for general purpose metallization of larger area substrates. As a result of extensive experimental studies we have made it economically feasible to use the sputtering technique to replace chemical plated through hole process on Al backed substrates.

Sputtering can be considered to be a molecular spray of metal taking place inside a vacuum system. This spray is generated when an inert gas plasma (such as Ar⁺) created by electric and magnetic fields, bombards a target made of the metal or alloy. A substrate in vacuum, held against this spray would get coated with the metal, by molecular bonding. Since a chemical reaction is not involved, the process is compatible between widely dissimilar materials. For example, processing of plated through holes on an Al backed PTFE substrate requires simultaneous coating of Cu, PTFE and Al. When the surfaces are prepared properly, this is easily achieved by sputtering Cu inside and around the holes to form a continuous seed film without cracks or blisters.

Sputtering is a "line of sight" coating process in the sense that the spray cannot reach around a corner. Chemical plating in a liquid medium is not subjected to this restriction as long as the liquid can freely flow along the surfaces. In processing heavy backed substrates by sputtering, this limitation can be overcome by the use of shallow "blind holes" just deep enough to expose the metal backing. (see Fig. 2.) In fact, from the machining point of view, blind holes are much easier to drill than through holes. For instance, a 25 mil diameter hole through a quarter inch Al backed substrate represents an aspect ratio of over 10, virtually impossible to achieve by conventional drilling methods. In order to overcome this problem it is general practice to counterbore much larger holes from the back of the substrate. On a typical 6" x 6" substrate having 100 plated through holes, this involves hours of machining time. Furthermore, the resulting honeycomb structure would greatly compromise the superior electrical, mechanical and thermal characteristics of the heavy metal backing.

Blind Hole Drilling

Precise Z axis control is required for blind hole drilling. Conventional printed circuit drilling machines do not generally have this feature. CNC milling equipment, on the other hand are not only bulky and expensive, but do not have the high rpm spindles necessary for attaining the smooth finish on soft materials. Specifically for the purpose of drilling blind holes we have developed a three axis computer controlled machine having a table travel of 12" x 12" adequate for most purposes. The incremental accuracy of the machine is ±1/2 mil on all the three axes. The small drilling head has a continuously variable speed of up to 60K rpm and can take standard drill/rout bits of 1/8" shank. The machine is also fitted with a microscope/video camera assembly that enables precise alignment of holes with respect to circuit features.

Based on Machine Automation Control Language, we have developed extensive software for drilling and milling blind holes and slots in a variety of shapes and sizes. Small blind holes (20 to 50 mil diameter, up to 50 mil deep) are drilled in the conventional way. Larger holes and slots require repeated "pecking" action. By the use of specially selected carbide tool bits and proper entry material, blind holes of consistently smooth surface finish can be obtained.

Sputtering

A dedicated sputtering system was constructed to accommodate substrates up to 12" x 12". The system is equipped with high efficiency magnetron sources operating at low (1 to 2 10^-3 torr) argon pressures, capable to sputtering the required thickness of Cu (20 - 40 µin) over the entire substrate in 30 minutes. Careful system design eliminates unwanted substrate bombardment and subsequent contamination of the deposited film. The Sputtering heads are so located that line of sight coverage occurs on the curved surfaces as well as the bottom of blind holes for an aspect ratio of up to 2. Chemical etching of the machined PTFE substrate is not necessary since the energetic Cu particles adhere to it by direct molecular bonding.

After sputtering the seed layer of Cu inside the blind holes, subsequent Cu plating is done in a standard acid bath, which has adequate throwing power to reach inside the blind hole. After the final plating, the circuit is patterned by conventional means of pholithography using dry film or liquid photoresist.

Performance Evaluation

In a microwave substrate a sputtered blind hole performs the same way as a chemically plated hole. The electrical properties are exactly the same since the bulk of conduction takes place through the electroplated Cu in both cases. A blind hole is mechanically stronger as the barrel of Cu is firmly anchored inside a cavity surrounded by the bulk metal. As part of the manufacturing process, substrates with blind holes are routinely subjected to solder reflow in hot oil or IR oven at temperatures of up to 250°C. The resulting thermal shock and cycling does not affect the integrity of the blind hole as confirmed by examining metallurgical cross section of such holes.

The DC resistance of a blind hole to ground is in the range of 1 milli ohm and is very difficult to measure with a high degree of accuracy. A dual probe method as shown in Fig. 3. is essential to ensure reliable readings. Even so, the absolute accuracy of measurements may have an uncertainty of as much as .5 milli ohm. Test pieces with blind holes have been subjected to thermal cycling between -65°C and 125°C. The resistance measurements made during such cycling show extremely high stability as shown in Fig. 4.

A detailed evaluation of possible modes of failure of blind holes when subjected to repeated thermal shock and temperature cycling, will be undertaken in the near future. At this point, it is not clear if the test conditions normally associated with printed wiring boards adequately represent the thermal fatigue effects, because of the bulk of heavy metal backing. New test criteria may have to be evolved after taking into consideration, the practical aspects of assembly and installation of heavy metal backed substrates. Tests are also planned to characterize the microwave properties of the blind holes, especially the effective inductance and the path length to groundplane.

Fig. 1. Anatomy of a sputtered blind hole

Fig. 2. Through hole versus blind hole

Fig. 3. Measurement of via ground resistance

Fig. 4. Effect of thermal cycling (-65°C TO 125°C) on via ground resistance

Sample 1

Dr. K. Ramachandran, Filtran Microcircuits Inc.
copyright © Filtran Microcircuits Inc. 1997

home | about | products | technology | design guide | contact | career opportunities